Radio communication systems are known to include a central controller, a bank of packet duplicators, a plurality of base sites, and a plurality of communication units. Each base site provides communication services to communication units located in its corresponding service coverage area. Each packet duplicator in the packet duplicator bank is coupled to each base site via a respective transmission link to provide interconnectivity between the base sites. In a similar manner, the central controller is coupled to each base site and each packet duplicator in the packet duplicator bank via a respective transmission link. The lengths of the transmission links between the packet duplicators and the base sites correspond to the distance between each base site and the packet duplicator bank.
Operation of the typical radio communication system occurs as follows. When a communication unit, such as a mobile or portable radio, initially desires to communicate with one or more other communication units, the communication unit transmits a channel origination request to a base site over a control channel. The channel request typically includes an identification of the requesting communication unit and identifications of any target communication units. Upon receiving the channel request, the base site forwards the request and an identification of the base site to the central controller. The central controller then assigns a packet duplicator from the packet duplicator bank to support the requested communication. The central controller typically selects the packet duplicator for a particular communication in a round robin manner to equalize the loading of the packet duplicators in the packet duplicator bank.
In addition, the central controller determines the approximate location of each target communication unit by paging each communication unit from one or more base sites. Upon receipt of a page, a target communication unit transmits an acknowledgment to a base site serving the service coverage area containing the responding communication unit. The base site receiving the acknowledgment forwards the acknowledgment and the base site identification to the central controller. By knowing which base site received a particular communication unit's page acknowledgment, the central controller knows which service coverage area the particular communication unit is most likely in and, hence, the particular communication unit's approximate location.
Once the central controller has identified the base sites serving the communication units involved in the requested communication, the central controller instructs each participating base site to allocate a traffic channel for the communication. The base sites then notify their respective communication units of their respective channel assignments and the communication system is ready for the transmission of the communication.
Upon being allocated a traffic channel, the requesting communication unit transmits a voice communication to its serving base site via the allocated traffic channel. The serving base site forwards the voice communication to the assigned packet duplicator in the packet duplicator bank. The packet duplicator replicates the voice communication and provides the replicated communication to all participating base sites via the respective transmission links connecting the packet duplicator to the base sites. Upon receiving the replicated communication, the base sites transmit the replicated communication to their respective communication units via the allocated traffic channels. The conveyance of the communication between the communication units continues in the above manner (i.e., one base site receives the communication and forwards it to the packet duplicator, where the communication is replicated and provided to the base sites via the transmission links ) for the duration of the communication. Upon termination of the communication, the central controller deassigns the packet duplicator and instructs the base sites to deallocate the traffic channels.
Depending on each participating communication unit's location in the system, the packet duplicator may have to convey the replicated communication to base sites via long transmission links. Since the packet duplicator bank is often centrally located in the system, the farther the packet duplicator bank is from the participating communication units, the more likely that long transmission links will have to be used between the packet duplicator bank and the base sites. Thus, if all the participating communication units are located in one or two service coverage areas located at the farthest edge of the system, long transmission links will be used for each conveyance between the serving base sites and the packet duplicator. The frequent use of long transmission links can become very costly for the user of the requesting communication unit or for the system subscriber (e.g., a taxi company) subsidizing the cost of establishing communications between the communication units (e.g., taxis). Similar to the situation in which the cost of making a local call is substantially less than the cost of making a long distance phone call over the public switched telephone network, the cost of establishing communications over short transmission links is substantially less than the cost of establishing communications over long transmission links. Thus, in the typical communication system, group communications involving communication units located a substantial distance away from the packet duplicator bank are, on average, very costly.
Therefore, a need exists for a method and system for establishing a communication between communication units using geographically distributed packet duplicators that utilize, on average, more short transmission links than long transmission links to convey communications between base sites and packet duplicators, thereby providing less costly communications for system users.